This study addresses the potentialities of flow cytometry in analyzing the composition of circulating hemocyte populations in the ascidian Halocynthia aurantium (Pallas, 1787) both using monoclonal antibodies (mAbs) against some human leukocyte conservative adhesion molecules and without mAbs. Flow cytometry, based on the assessment of forward and side scattering revealed five hemocyte populations. From the wide panel of antibodies against human leukocyte adhesion molecules (CD15, CD29, CD34, CD54, CD62L, CD62P, CD90, CD94, CD117, CD 166), only two mAbs (against CD54, CD90) displayed cross-reactivity with the H. aurantium hemocyte surface antigens. Distribution patterns of these antigens across the hemocyte populations have been analyzed.